Spur-of-the-moment
Part of speech: adjective
Definition: Spontaneous; impromptu.
Unprepared
Part of speech: adjective
Definition: Not prepared; caught by surprise.
Example sentence: It is inconceivable that even the gang who runs Russia would be willing to take on war, but one always has to remember that there seemed to be no reason in 1939 for Hitler to start war, and yet he did, and he started it with a world practically unprepared.